Dielectric ceramic materials have been used extensively for dielectric resonators, MIC dielectric substrates, waveguides and the like in a high frequency region such as of microwave and millimeter-wave. The dielectric ceramic materials are generally required to have the following three principal properties:
(1) A high dielectric constant to meet a demand for size reduction of devices, because the wavelength is reduced to 1/.epsilon.r.sup.1/2 in a dielectric; PA0 (2) A small dielectric loss in the high frequency region, i.e., a high Q value; and PA0 (3) A small change in resonant frequency with respect to a temperature change, i.e., a stably low temperature dependence of the dielectric constant.
Exemplary dielectric ceramic materials hitherto known include oxide ceramic materials such as BaO--TiO.sub.2 based materials, BaO--REO--TiO.sub.2 (wherein REO is an oxide of a rare earth element) based materials and MgTiO.sub.3 --CaTiO.sub.3 based materials (see Japanese Unexamined Patent Publications No. 61-10806 (1985), No. 63-100058 (1988) No. 60-19603(1985)).
Although the BaO--TiO.sub.2 based materials have a dielectric constant .epsilon.r of 37 to 40 and a high Q value on the order of 40,000, it is difficult to achieve a resonant frequency temperature coefficient .tau.f of zero in a single phase (the resonant frequency temperature coefficient .tau.f represents the temperature dependence of the dielectric constant). Further, these materials suffer from a great change in the dielectric constant and the temperature dependence of the dielectric constant with respect to a change in the composition thereof. Therefore, it is difficult to stably reduce the resonant frequency temperature coefficient .tau.f while ensuring a high dielectric constant and a low dielectric loss.
Known as the BaO--REO--TiO.sub.2 based materials are BaO--Nd.sub.2 O.sub.3 --TiO.sub.2 based materials and BaO--Sm.sub.2 O.sub.3 --TiO.sub.2 based materials. These materials have a dielectric constant .epsilon.r of 40 to 60, and some of them have a resonant frequency temperature coefficient .tau.f of zero. However, the materials have a low Q value on the order of 5,000 or lower.
The MgTiO.sub.3 --CaTiO.sub.3 based materials have a high Q value on the order of 30,000, and some of them have a resonant frequency temperature coefficient .tau.f of zero. However, the materials have a low dielectric constant on the order of 16 to 25.
Therefore, none of the above materials perfectly satisfy the aforesaid three principal properties required of the dielectric material for high frequency applications.